Now a team of 10 trained pigeons is taking to the skies strapped with 25-gram sensors to measure the harmful emissions not always visible to the naked eye -- and, rather aptly, are tweeting the results.

Londoners can ask their feathered friends to track nitrogen dioxide levels in their area by tweeting @PigeonAir. The clever creatures, with a little help from researchers at Plume Labs, will then tweet back their readings, ranging from "moderate" to "extreme."

One of the pigeons -- which have names such as Norber, Coco and Julius -- will also be strapped with a GPS tracking device. A vet will be on hand to monitor the birds' well-being over three days of flights.

The winged patrol was originally dreamed up by Pierre Duquesnoy and Matt Daniels of marketing agency DigitasLBi as part of Twitter's #PoweredByTweets competition last year, winning the "Solve a Problem" category.

The patrol aims to boost a much larger campaign by Plume Labs to recruit 100 Londoners to carry out their own pollution monitoring -- though this one will be on the ground.

"Millions of people die every year around the world from air pollution -- it's basically a pandemic, but we have a hard time realizing this because it's largely invisible," Romain Lacombe, CEO of Plume Labs, told CNN.

"So the Pigeon Patrol has been about making the invisible, visible. And there's a nice parallel there -- we don't always think about the fact we breathe about 20 (kilograms)of air every day.

"Much in the same way that pigeons themselves are all around us, and we rarely notice them."

Though with their fancy new backpacks on, these pigeons will be hard to ignore -- just keep them off the catwalk.